Traditional high-speed weighing devices, such as linear variable differential transformers and piezoelectric strain-gauge and force-restoration devices require contact with the substance to be weighed. They also require a vibration-free environment. These requirements are sometimes inconvenient to meet, and are avoided by capacitive weighing methods which are a relatively recent development. Unlike conventional weighing devices, capacitive weighing does not measure force; but rather, it measures the product of mass and other electrical constants of the material to be weighed. The theory is based on the change in capacitance effected by placing a small object in the space between the plates of a parallel-plate capacitor, the capacitance changing in accordance with the formula: EQU .DELTA.C=[m(D-1)]/.rho.
where .DELTA.C is the change in capacitance, m is the mass of the object, D is the dielectric constant of the object, and .rho. is its density.
U.S. Pat. No. 4,223,751 discloses capacitive weighing apparatus for determining whether the mass of a capsule lies within a predetermined range. The apparatus includes a capacitive sensing transducer having a pair of parallel plates to which a rapidly changing electric field is applied. A non-conductive tube placed between the plates defines a conduit for guiding capsules into the region between the plates. The tube thus establishes the attitude of the capsules as they are pneumatically propelled through the plates in such a way that only one capsule passes between the plates at any one time. The output of an amplifier electronically coupled to the plates is proportional to capacitive changes produced by the passage of a capsule. Electrical circuitry responsive to the output of the amplifier determines whether the mass of the capsule is within a predetermined range. Thus, the weighing apparatus of this patent is limited to weighing samples having a defined envelope presented to the apparatus in a predetermined attitude.
U.S. Pat. No. 3,979,581 discloses a capacitive system in which material to be weighed is moved along a predetermined path to a weighing station, where the material lies in a capacitive cell forming part of a high-frequency oscillator circuit. When the material is within the capacitive cell, its capacitance and the attenuation of the high-frequency oscillator circuit at resonance are both measured. The mass of the material in the capacitive cell is calculated by means of an nth-degree polynomial, whose constants are stored in a computer, and whose variables are the signals corresponding to the resonant attenuation and capacitance. The computed mass value is used to control the mass of the material moving along the path.
The known prior art is limited to weighing samples whose presence in a capacitive cell produces a substantial change in capacitance of the cell because of the relatively large signal-to-noise ratio produced. Where the sample is so small that it produces a relatively small change in capacitance, the noise component in the output of the transducer becomes significant; and thus, spurious results are likely when the sample has a relatively small mass. In addition, the prior art requires the presentation of bounded samples in a predetermined attitude within the transducer. As a consequence, conventional weighing apparatus is unreliable for making dynamic measurements of small amounts of powdered samples which flow through a transducer and have no fixed boundary or attitude relative to the transducer, or which may not be entirely within the transducer at any time during transit therethrough.
It is, therefore, an object of the present invention to provide a new and improved capacitive weighing apparatus which overcomes or substantially ameliorates the above-described problems with conventional capacitive weighing apparatus.
Still another object of the present invention is to provide an improved capacitive weighing device which provides correction for particles of powdered samples which accumulate on the transducer over time, and for temperature drift of the capacitive weighing device over time.